Two demon brothers, sons of Nikumbha.
They had received the boon of Brahma that they can only die if they kill each other. Since they considered this to be impossible, they believed they had thus achieved invincibility. They became a plague.
The Gods sent the beautiful apsara Tilottama. The brothers got into jalousy and quarrel and finally killed each other.
Vyasa told this to the Pandavas as a warning. They were five and had one wife, Draupadi.